This invention relates generally to repetitive digital sampling circuits and in particular to method and apparatus for generating a repetitive digital sampling rate with improved time accuracy.
Digital sampling is a process by which analog signals may be convened to digital representations. The analog to digital converter (ADC) is a commercially-available device for digitizing signals. Used in test and measurement applications, the ADC may be used to digitize a complete waveform. The most basic method of digitizing a waveform involves converting the instantaneous voltage level of the signal present at the input of the ADC into a digital sample. A sample-and-hold circuit (S/H) may be employed ahead of the ADC to capture the instantaneous voltage value at a desired instant in time and hold the voltage value for the ADC to measure. Conventional digital circuitry, such as microprocessors and memory devices, can be used to receive the digital samples and store them collectively as a time record for later retrieval and reconstruction of the input signal.
In order to accurately reconstruct the input signal from the stored samples, the effective digital sample rate becomes a concern. Capturing a waveform involves sequentially sampling and storing the instantaneous voltage levels of the signal at a rate high enough to reconstruct the waveform from the stored digital samples. The minimum rate necessary to accomplish this is generally determined by the Nyquist rate, which is understood to equal two times the maximum bandwidth of the signal. As long as the ADC has the ability to sample fast enough to reconstruct a signal of interest, the process of digital sampling can be accomplished with real-time sampling. In real-time sampling, the ADC sequentially samples the input signal and the digital samples are then stored in corresponding memory locations. However, an ADC with an adequate real-time sample rate may not be available for higher frequency input signals.
The effective sample rate of ADC's has been extended so that higher frequency signals may be digitized using a process commonly referred to as repetitive digital sampling. The basic requirements for repetitive digital sampling are that the sampled signal be repetitive and that a stable starting point on the signal be known. For signals that meet this requirement, repetitive sampling may be employed to obtain an equivalent digital sampling rate many times higher than the real-time digital sampling rate by digitizing selected points of the signal over multiple periods. Repetitive digital sampling circuits employ sophisticated timing circuits which determine selected points on the signal at precisely calculated time intervals from the starting point of the repetitive signal. A number of samples at selected points on the signal are collected and placed in corresponding locations in a digital acquisition memory. A composite representation of the repetitive signal may then be constructed from the stored digital samples. The particular order in which particular portions of the signal are digitized is not critical. The time resolution of the reconstructed signal is a direct function of the time resolution of the timing circuits employed to determine the time delay.
Repetitive sampling techniques are employed in electronic test equipment such as digital storage oscilloscopes (DSO's) to obtain high equivalent sampling rates in a manner as previously described, often with time resolutions of one nanosecond or less. DSO's typically use timing circuits to generate a series of precisely timed sample pulses which are directed to a S/H circuit to capture a voltage level at a selected time delay from the starting point. The digital samples are placed in digital memory at locations corresponding to their time interval from the starting point at the sample rate governed by the ADC. The timebase circuit necessary to generate the sample pulses at the time interval accuracy needed tend to be expensive and complex because high speed clocks and digital counters operating at the equivalent sampling rate are required to keep track of the selected time delay.
Closely associated with the time resolution provided by a repetitive digital sampling circuit is its time interval accuracy in producing sampling pulses. Ideally, all collected digital samples occur at time intervals of equal spacing on the signal being measured. Time interval accuracy is of critical importance to the overall performance of electronic test equipment employing repetitive digital sampling because without the equal sample spacing in time, distortion and errors in the sampling process arise in the reconstruction of the sampled signal. Thus, digital sampling circuits that provide high time resolution sampling pulses must do so with substantially equal time interval spacing to be acceptable for use in electronic test equipment.
In an application for a portable, electronic test instrument in which a high frequency, repetitive signal is to be measured, the needs for low power consumption and low component cost and complexity necessitated another approach to implement a digital sampling circuit with a high equivalent sample rate and with a time interval accuracy level to support that sample rate. It would therefore be desirable to provide such a digital sampling circuit using relatively low cost, commercially-available electronic components.